The topic of small antennas has been of prolonged interest and goes back more than half a century. Using the area of the substrate more effectively in microwave circuits, as part of a general trend in monolithic circuit integration and antenna invisibility for certain applications, has been among the major motivations. On the other hand, in the radio communication, where the line of sight communication is not generally possible, the UHF-VHF frequencies should be used. At these low frequencies, the size of even a single half wave dipole antenna is preclusive in many mobile and wireless applications.
The subject of antenna miniaturization is not new. The literature concerning this subject date back to the early 1940's. It has been shown that the Q factor of the equivalent circuit for each spherical mode can be expressed in terms of the normalized radius (a/λ) of the smallest sphere enclosing the antenna and also the Q of the lowest order mode is a lower bound for the Q of a single resonant antenna. A similar procedure is used, for characterization of a small dipole antenna using cylindrical wave functions. Then a cylindrical enclosing surface is used which produces a tighter lower bound for the Q of small antennas with large aspect ratios such as dipoles and helical antennas. Qualitatively, these studies show that for single resonant antennas, the smaller the maximum dimension of an antenna is, the higher the Q of the antenna or equivalently the lower the bandwidth of the antenna. However, the studies do not provide a description of the process for practicing the miniaturization methods, antenna topology, or impedance matching.
Normally, there is a compromise between the size, efficiency and bandwidth of the antenna. It is known to address this subject by expanding fields of an arbitrary small antenna enclosed in a sphere, using spherical eigen-functions expansion. The Q of the antenna, which is by definition the ratio of the stored energy to the radiated power, can be related to the Q of each eigen-mode. This approach introduces a lower bound on the Q of the antenna. The calculated Q is a function of radius of the sphere or correspondingly the largest dimension of the antenna. On the other hand, a lower bound on Q in some senses is an indication of an upper limit on the antenna bandwidth. There are two ways to achieve miniaturization. One is to use a high permittivity substrate and the other is to exploit the substrate area in two dimensions by changing the topology of the antenna
With the advent of wireless technology and ever increasing demand for high data rate mobile communications the number of radios on mobile platforms has reached a point that the available real estate for these antennas has become a serious issue. Similar problems are also emerging in the commercial sector where the number of wireless services planned for future automobiles, such as FM and CD radios, analog and digital cell phones, GPS, keyless entry and etc., is on the rise. Considering wave propagation where line-of-sight communication is an unlikely event, such as in an urban environment or over irregular terrain, carrier frequencies at HF-UHF band are commonly used. At these frequencies there is considerable penetration through vegetation and buildings, wave diffraction around obstacles, and wave propagation over curved surfaces. However at these frequencies the size of efficient antennas are relatively large and therefore a large number of such antennas may not fit in the available space without the risks of mutual coupling and co-site interference. Efficient antennas require dimensions of the order of half a wavelength for single frequency operation. To cover a wide frequency range, broadband antennas may be used, however, dimensions of these antennas are comparable to or larger than the wavelength at the lowest frequency. Besides, depending on the applications, the polarization and the direction of maximum directivity for different wireless systems operating at different frequencies may be different and hence a single broadband antenna may not be sufficient. It should also be noted that any type of broadband antenna is highly susceptible to electronic jamming techniques. Variations of monopole and dipole antennas in use today are prohibitively large and bulky at HF through VHP.